First things off... I used to have a Logitech G700 and I had really bad aim, for the longest time I thought it was me but turns out my G700 was faulty and the sensor was a bit broken, you can test this in paint, start drawing thin lines and see how jerky they come out, mine were incredibly jerky which caused my aim to go all over the place.
Assuming your G700 is fine you're still dealing with a laser sensor and they are really not that good and also very picky about the surface they are on. I strongly suggest moving to optical sensors.
Hardware wise aside... you need to set a baseline for your sensitivity and work from there, I suggest setting 800 DPI for 1080p, don't change your DPI per game and not even for Windows usage.
Start measuring your sensitivity in game by cm per 360, you can track this by measuring how many cm's it takes for you to do a full 360 turn. This is especially useful to keep the same sensitivity across all games.
For Overwatch I suggest starting out at around 7 or 6 sensitivity, lower is better but if you are used at faster sens then it is harder to make the switch but you end up getting used to it. Most pro's use 800 DPI and 5 in game.
In the end all it matters is that you can track your targets comfortably. When playing and practicing don't be too hard on you if you feel you can't track properly, it takes a long time even after small sensitivity changes for you to start getting used to it, most important thing is for you to realize if you are over aiming or under aiming when you try to track the enemy.